Progress In Electromagnetics Research, PIER 77, 137–148, 2007

A MODIFIED MICROSTRIP-FED TWO-STEP TAPEREDMONOPOLE ANTENNA FOR UWB AND WLANAPPLICATIONS

R. Zaker, Ch. Ghobadi, and J. Nourinia

Department of Electrical EngineeringUrmia UniversityUrmia, Iran

Abstract—This paper presents a novel modified Printed TaperedMonopole Antenna (PTMA) for ultra wideband (UWB) wirelesscommunication applications. The proposed antenna consists of atruncated ground plane and two-tapered radiating patch separatedby a slot (air gap) of different slopes, which provides a widebandbehavior and relatively good matching. Moreover, the effects of amodified T-shaped slot inserted in the first tapered patch, on theimpedance matching is investigated. The antenna has a small area of23 × 26.5 mm2 and offers an impedance bandwidth as high as 100%at a centre frequency of 7.2 GHz for S11 < −10 dB, which has anarea reduction of 15% and a frequency bandwidth increment of 72%with respect to the previous similar antenna. The presented antennacovers the 5.2/5.8 GHz WLAN and 5.5 GHz WIMAX operating bands.Numerical analysis using Ansoft HFSS and measurement results is alsopresented in the paper.

1. INTRODUCTION

Wireless communications have been developed widely and rapidlyin the modern world specially during the last decade. The futuredevelopment of the personal communication devices will aim to provideimage, speech and data communications at anytime, and anywherearound the world. This indicates that the future communicationterminal antennas must meet the requirements of multi-band or wide-band to sufficiently cover the possible operating bands. However, thedifficulty of antenna design increases when the number of operatingfrequency bands increases and cover an octave or more. In addition,for miniaturizing the wireless communication system, the antenna must

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also be small enough to be placed inside the system [1]. To achievethis, planar monopole antennas are good candidates for wide-bandapplications, as they exhibit wide impedance bandwidth, compact andsimple structure, and ease of construction [1–8, 13]. Moreover, theomnidirectional radiation properties of monopole antenna make themvery suitable for base-station and for indoor applications.

Recently, several microstrip slot antennas [11, 12] and planarmonopole geometries such as circular, square, rectangular, elliptical,hexagonal and pentagonal, have been analyzed, providing wideimpedance bandwidth. One of the best antennas in the last decadeis the tap monopole antenna. The planar tap monopole antennashave been adopted and studied extensively for UWB communicationsystems because of their many appealing features such as simplestructure, small size, wide impedance bandwidth and omnidirectionalradiation patterns [1]. In [6, 7], two new small wideband planarmonopole antennas with truncated ground plane using an L-shapednotch in the lower corner to achieve the maximum impedancebandwidth were proposed. In [1] a small printed tap monopole antenna(PTMA) for UWB wireless communication applications was designedand its numerical analysis was presented. In this design, a slit, taperedtransition and two-step staircase notch are implemented to obtain theultra wide bandwidth of the antenna. This antenna is a good candidatefor hand-held UWB applications. In [5], a novel antenna topology waspresented based on the PTMA. In this design, The antenna structureconsists of a tapered radiating element fed by a microstrip line anda slot in the radiating element and in the ground plane, which yieldsa wideband behavior with a relatively good matching. The proposedantenna, with small size of 25.0 × 28.5 mm2, was designed to operateover the frequency band between 3.11 and 7.30 GHz for S11 < −10 dB(VSWR ≤ 2).

In this paper, a novel modified Printed Tapered MonopoleAntenna (PTMA) for ultra wideband (UWB) applications is presented.Our new antenna, consists of a truncated ground plane and two-taperedradiating patch separated by a slot and with different slopes, whichprovide the maximum impedance bandwidth for UWB (3.1∼10.6 GHz)[10], WLAN (5.2/5.8 GHz) and WIMAX (5.5 GHz) applications. Theproposed structure is designed based on the antenna presented in [5]but with a smaller size, lower cost and higher frequency bandwidth.In this paper, we investigate the effects of various slopes for the two-tapered patch and also insertion of a modified inverted T-shaped slotin the first tapered patch on the frequency bandwidth and impedancematching. The results of this paper are obtained from Ansoft HFSSsimulations [9], which are based on the Finite Element Method (FEM).

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2. ANTENNA DESIGN AND PARAMETERS

Figure 1 shows the structure and dimensions of the proposed antenna,whose conductor is fabricated on an inexpensive FR4 substrate withthe dielectric constant of εr = 4.4 and the substrate thickness ofh = 1.6 mm. The antenna shape and its dimensions were firstdesigned based on the antenna presented in [5] and modified usingthe Ansoft High Frequency Structure Simulator (HFSS). Then theoptimal dimensions were determined from experimental adjustment.The dimensions of the designed antenna, including the substrate, isL × W = 26.5 mm × 23 mm, or about 0.4λ × 0.35λ at 4.6 GHz. A50 Ω microstrip feedline with width of W1 = 3.4 mm and length of4 mm, is used to feed the antenna centrally from the bottom edge ofthe rectangular strip. The antenna is symmetrical with respect to thelongitudinal direction.

Figure 1. Configuration and parameters of proposed planar tapmonopole antenna (PTMA) with two-tapered patch and truncatedground plane.

The basic antenna structure consists of a truncated ground planeand two-tapered radiating patch with different slopes separated by aslot of width (Ws) = 0.2 mm and length (W4) = 16.8 mm. In thisstructure, the slot is added in the tapered radiating element, becauseprovides a wideband behavior with a relatively good matching [5], alsoon the other side of the substrate, a conducting ground plane of width(W ) = 23 mm and length (d2) = 7.8 mm is placed. The modified

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truncated ground plane acts as an impedance matching element tocontrol the impedance bandwidth of a square monopole [6], because ithelps matching the patch with the two-step feedline in a wide rangeof frequencies. This is because the truncation creates a capacitiveload that neutralizes the inductive nature of the patch to producenearly-pure resistive input impedance [1]. The dimensions of the notch(Wg × dg) embedded in the truncated ground plane are importantparameters in determining the sensitivity of impedance matching. Inaddition, to achieve good wideband matching of the proposed antenna,a separation d = d1 − d2 (= 0.1 mm) between the two-tapered patchand the notch in the ground plane is used. The introduction of a bevelincreases the upper-edge frequency, and control of this frequency ispossible by adjusting the bevel angle [8]. Therefor the two angles αand β (the slopes of tapered patch 1 and 2, respectively) are otherimportant factors in determining frequency bandwidth and impedancematching.

3. NUMERICAL ANALYSIS AND RESULTS

The important parameters of proposed antenna are α, β, W2 (the widthof second step of the feedline), d = d1 − d2, Ws and ds, all in the toplayer and also Wg and dg in the bottom layer. The parameters of thisproposed antenna are studied by changing one parameter at a timeand fixing the others. The constant factors of this proposed antennaare: W1 = 3.4 mm, W3 = 5.0 mm, W4 = 16.8 mm, Ws = 0.2 mm,ds = 9.5 mm, Wg = 4 mm and dg = 3.8 mm.

The properties of three general cases specified by values of α andβ are summarized in Table 1. Three particular cases where either orboth of α and β is zero are specified in Table 2. We recall that in allof these cases W3 and W4 are constant.

Table 1. Three general cases of proposed antenna with different anglesof α and β.

Case deg ) ( deg ) 1 13 ° 13 °

2 31 ° 7 °

3 20 ° 13 °

α β (

The simulated VSWR curves for each case of Table 1 are plottedin Fig. 2. From the simulation results in Fig. 2, we found out that thefrequency bandwidth and impedance matching of case 2 (α = 31 andβ = 7) are better than those of cases 1 and 3.

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Table 2. Three particular cases of proposed antenna with differentangles of α and β.

Case ( deg ) ( deg ) 1 0 ° 13 ° 2 31 ° 0 ° 3 0 ° 0 °

α β

Figure 3 shows the effect of α and β parameters (Table 2) in thetwo-tapered patch on the frequency bandwidth, impedance matching,higher and lower operating frequency. From the simulation resultsof Fig. 3, it is found that the optimized parameters of α and βare 31 and 0 (case 2), respectively. In the second case, the lowerfrequency is lowered and the upper frequency is markedly increased.The electromagnetic field coupling between two-tapered patches iseffectively changed by varying the size of α and β. Moreover fromFigures 2 and 3, consequently we conclude that the parameters of α andβ are the critical factors to determine the upper and lower operatingfrequencies and impedance matching.

Figure 2. Simulated VSWR characteristics of the proposed antennawith different values of α and β (Table 1) and W3 = 5.0 mm, W4 =16.8 mm.

Another important parameter of this structure is the width of thesecond step (W2) in the feedline. Figure 4 shows the effects of W2 onthe impedance matching when W1 = 3.4 mm and W3 = 5.0 mm. Wehave observed that the impedance matching is effectively changed by

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the varying the size of W2, specially in the middle band. The optimizedvalue of W2 is chosen as 3 mm.

Figure 3. Simulated VSWR characteristics of the proposed antennawith different values of α and β (Table 2).

Figure 4. The effect of various W2 (the width of the second step inthe feedline) on VSWR.

At the end of our design procedure the final dimensions aresummarized in Table 3. Figure 5, presents the photograph of a realizedprinted PTMA on an FR-4 substrate with SMA connector.

Figure 6 shows the measured and simulated VSWR characteristicsfor the proposed antenna. As shown in Figure 6, there existsa discrepancy between measured data and the simulated results

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Table 3. Optimized parameters (mm).

Parameters Values (mm) Parameters Values (mm) L W 26.5 23 d = d1-d2 0.1

W1 3.4 d2 7.8 W2 3.0 Ws 0.2 W3 5.0 ds 9.5 W4 16.8 Wg 4.0

31 ( deg ) dg 3.8 13 ( deg )

αβ

× ×

Figure 5. Photograph of the realized printed PTMA.

Figure 6. Measured and simulated VSWR for the proposed antenna.

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at 4.5 GHz and 5.2 GHz. The measured impedance bandwidth iswider than simulated one. In order to achieve the accurate VSWRcharacteristics for the designed antenna, it is recommended thatthe manufacturing and measurement process need to be performedcarefully. The fabricated antenna satisfies the 10 dB return loss(VSWR < 2) requirement for 5.2/5.8 GHz of WLAN and 5.5 GHz forWIMAX. Figure 7 shows the antenna gain from 3.5 to 10 GHz for theproposed antenna. The maximum gain variation is less than 2.2 dBwith the peak antenna gain of about 6.0 dBi.

Figure 7. Simulated antenna gain of the proposed antenna.

Figure 8 shows the measured radiation patterns including theco-polarisation and cross-polarisation in the H-plane (x-z plane).It can be seen that the radiation patterns in x-z plane are nearlyomnidirectional for the four frequencies, specially 5.2 and 5.8 GHz(WLAN).

A modified inverted T-shaped slot is inserted in the first taperedpatch of the proposed antenna as displayed in Figure 9. Two suchslots with different sizes are specified in Table 4 as case 1 and 2.Figure 10 shows the effects of T1 and T2 on the impedance matchingin comparison with the same antenna without slot. It is found out

Table 4. Different values of T1 and T2 (parameters of T-shaped slot).

Case T1 (mm) T2 (mm) 1 5.5 4.5 2 2.0 9.0

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( a ) ( b )

( c ) ( d )

____ co-polar _ _ _ cross-polar

Figure 8. Measured radiation patterns (H-plane, x-z plane ) of theproposed antenna at: (a) 4.2 GHz, (b) 5.2 GHz, (c) 5.8 GHz, and (d)8.6 GHz.

Figure 9. Geometry of the tapered patch 1 with a modified T-shapedslot.

146 Zaker, Ghobadi, and Nourinia

that the impedance bandwidth increases when the length T1 decreasesand T2 increases (case 2). On the other hand, the lower frequency isinsensitive to the change of T1 and T2. The inserting of the modifiedT-slot, divides the surface current paths on the first tapered patch,therefore the electromagnetically coupling value between two-taperedpatches is increased. Note that here the coupling in the right and lefthands of the T-slot between two patches is considered.

Figure 10. Simulated VSWR characteristics for the proposed antennawithout slot and two cases 1 and 2 with slot as shown in Table 4.

4. CONCLUSION

A simple printed monopole antenna with two-tapered patch separatedby a slot and truncated ground plane has been presented. The useof two-tapered patch with different slopes, a slot between them, ourmodified feeding structure and a slot in the ground plane has increasedimpedance bandwidth.

A better impedance matching can be achieved by insertion amodified T-slot on the first tapered patch and carefully choosing itsparameters. The proposed antenna has the frequency band of 3.6to over 10.8 GHz for VSWR less than 2.0, which has a bandwidthincrement of 72% with respect to the previous similar antenna. Thisantenna covers the 5.2/5.8 GHz WLAN bands and 5.5 GHz WIMAXband. Experimental results show that the proposed antenna can be agood candidate for hand-held UWB, WLAN and WIMAX applications.

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ACKNOWLEDGMENT

The authors are thankful to Iran Telecommunication Research Center(ITRC) for its financial support and also the Antenna Lab of theKhaje Nasir Toosi University of Technology (Tehran, Iran) where theproposed antenna has been tested.

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